Fluorescent hydrogels with emission enhancement and CPL-activity depending on gelation states

Abstract

The photoluminescence properties of fluorescent hydrogels are strongly influenced by the supramolecular organization of the chromophores, which makes their modulation nontrivial. In this work, a series of fluorescent hydrogels with different emission colors were prepared by co-assembly of organic dyes with an amphiphilic peptide gelator. Depending on the two distinct gelation states of the peptide, the obtained fluorescent hydrogels exhibited different emission properties. The hydrogels prepared at lower alkalinity (Gel 1) could enhance the emission of the doped dyes that carry cationic groups (namely rhodamine B and sulforhodamine B), while those prepared at higher alkalinity (Gel 2) caused the emission quenching of the dyes. Based on the structural characterization of two types of hydrogels, it is speculated that nanowire aggregates in Gel 1 function as a template to direct the organization of co-assembled dye molecules through electrostatic interactions, and the emission is enhanced due to the restriction of molecular motion. In contrast, the less organized aggregates in Gel 2 allow the dye molecules to self-assemble through π–π stacking which leads to emission quenching. Due to the homochirality of the amino acid residues in peptide gelators, the aggregates of Gel 1 exhibit supramolecular chirality which is confirmed by CD spectroscopy. Consequently, optical activities including CD activity and CPL properties were also observed for the fluorescent hydrogels based on the Gel 1 matrix, and they originated from the chiral arrangement of dye molecules directed by the ordered anionic L-serine residues on the periphery of nanowires.